sys: net: Initial import of a UART network interface

~~Depends on #1448~~

Based mostly on (and aimed to replace) [`sys/net/network_layer/sixlowpan/bordermultiplex.h`](https://github.com/RIOT-OS/RIOT/blob/master/sys/net/network_layer/sixlowpan/border/bordermultiplex.h)
Can we get rid of this, too? Otherwise I'd still like a comment.